Offshore oil production has only moved into the very deep waters in recent years. Most seismic surveys in deep water have been carried out with seismic cables towed behind a vessel of opportunity. Newer seismic techniques lay relatively short cables on the sea floor having a total length of only 3 to 6 kilometers. Some seismic techniques require permanently installing seismic arrays on the sea floor to monitor the depletion of deepwater hydrocarbon reservoirs.
Ocean bottom cable technology utilizes acoustic detectors that are deployed at fixed locations at or near the ocean bottom. An acoustic source is towed near the ocean surface, and it imparts acoustic energy into the water that is reflected from geological strata and interfaces below the ocean bottom and that is measured by the acoustic detectors. The measured signals are, as typical in the seismic prospecting field, indicative of the depth and location of the reflecting geological features.
Typically, the ocean bottom detectors include both a geophone and a hydrophone, for recording both pressure and velocity information. This dual-sensor approach can help eliminate ghost and reverberation effects.
Ocean bottom cable detectors are often advantageous, as compared to towed detectors, in performing surveys in crowded offshore regions, such as may be encountered near offshore drilling and production platforms (which are often present, of course, near hydrocarbon reserves). The cost of each pass of the source vessel through the survey region is also relatively low when using ocean bottom detector cables, considering that the source vessel does not need to tow hydrophone streamers.
In seismic exploration over a body of water 11 (see FIG. 1), a seismic survey ship 12 is equipped with an energy source S for taking seismic profiles of a subsea underground structure. The act of taking profiles is often referred to as “shooting” due to the fact that explosive devices have been commonly used for many years as energy sources. The energy source is designed to produce compressional waves that propagate through the water to the seabed 17 and into the underwater formation. As the compressional waves C propagate through the subsurface, they strike interfaces 18 between formations, commonly referred to as strata, and reflect back through the earth and water to a receiver R. The receiver typically converts the received waves into electrical signals which are then processed into an image that provides information about the structure of the subterranean formation. FIG. 1 shows receivers R on the seabed 17.
One of the most common energy sources S is an air gun that discharges air under very high pressure into the water. The discharged air forms a pulse which contains frequencies within the seismic bandwidth.
Another energy source which is frequently used is a marine vibrator. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free from spurious bubbles.
Receivers R having hydrophones convert pressure waves into electrical signals that are used for analogue or digital processing. The most common type of hydrophone includes a piezoelectric element which converts physical signals, such as pressure, into electrical signals.
In bottom-cable seismic recording, a combination of pressure sensitive transducers, such as hydrophones, and particle velocity transducers, such as geophones are deployed on the sea bottom 17.
While geophones are typically used in land operations where metal spikes anchor the geophones to the ground to ensure fidelity of geophone motion to ground motion, geophones cannot be economically anchored in marine applications. Therefore, cylindrical, gimballed geophones are attached to the bottom-cable. After the cable is deployed from the seismic survey ship, the geophones simply lie in contact with the marine bottom 17 where they fell. The gimbal mechanism inside the cylinder assures that the geophone element mounted therein is oriented vertically for proper operation.
It is clear from the foregoing discussion that a variety of seismic equipment and techniques may be used in an attempt to accurately plot the subsea underground formation. Regardless of which technique or combination of equipment is used, each offers certain advantages and disadvantages when compared to one another. For instance, gathering seismic data with a towed streamer in areas populated with numerous obstacles, such as drilling and production platforms, can be difficult or even impossible because the streamer may strike one of the obstacles and tear loose from the towing vessel. Such an event represents an extremely costly loss.
By way of further background information, separation of pressure (compressional or P) wave and shear (or S) wave components by signal processing techniques is known. An example of such separation is described in Kendall, et al., “Noise analysis, using a multi-component surface seismic test spread”, presented at the 63rd Annual Meeting of the Society of Exploration Geophysicists (1993). This approach performs multi-component rotation analysis at individual receiver positions for each in a series of ray emergence angles, until one is found that maximizes the energy for P and S waves simultaneously.
A remotely operated vehicle (ROV) is a robotic tool 20 for performing underwater work. Many underwater operations (such as drilling and production of oil and gas, installation and maintenance of offshore structures, laying and maintaining underwater pipelines, etc.) require the use of an ROV or robotic tooling.
The deployment of an ROV is typically achieved by launching the unit from a floating host platform 13, a dynamically positioned marine vessel or ship dedicated, specifically for the purpose of supporting an ROV (e.g., an ROV support vessel or “RSV”), or any such surface vessel with sufficient size and characteristics that provide a suitably stable platform for the launching and recovery of an ROV.
Operations of an ROV are limited according to the distance that the ROV can travel from the host platform 13 as well as by restrictions in operating periods due to the collateral activities of the host platform.
In the case of dedicated vessel deployment, such as an RSV, significant costs are associated with operation of a fully founded marine vessel and its mobilization to and from the ROV work site. Typically, a dedicated RSV may have a crew of twenty and a considerable cost not directly related to the operation of the ROV.
ROV operation and monitoring is controlled from the host platform 13 or RSV by means of an umbilical line between the host platform or RSV and the ROV. The operational distance of the ROV is directly related to the length of the umbilical line. That line often includes a control container 21 or Tether Management System.
Therefore, the deeper the water, the longer it takes an ROV to travel from the surface to the bottom. Moreover, the power supply of the ROV is limited. When the ROV has to make many trips to the seafloor, it means that the ROV, relatively speaking, wastes more energy travelling from the surface 11 to the sea bottom 17 than it expends in doing useful work. Relatively speaking, the crew of the ship spends more time waiting for work than doing useful work. Clearly, work on the seafloor at deep depths means more expense overall.
This problem has existed for some time. Considerable effort has been made, and significant amounts of money have been expended, to resolve this problem. In spite of this, the problem still exists. Actually, the problem has become aggravated with the passage of time because oil and gas are now being found in deeper and deeper parts of the world's oceans.